Competitive interactions between exotic and native grasses under changing precipitation and increased nitrogen deposition in the Colorado Front Range

Background/Question/Methods

Over the last several decades, the Front Range of Colorado has been experiencing increased invasion of exotic cool season plants. Among the most problematic of these species is Bromus tectorum, an annual grass with severe impacts on arid and semi-arid ecosystems of the Intermountain West but not historically abundant in the Front Range. The spread of B. tectorum may be due to its increased competitive advantage over native species under (documented) changing climatic and edaphic conditions. We tested this hypothesis using an in-situ factorial experiment with three spring removal treatments (removal of B. tectorum, removal of the dominant early season native grass, Pascopyrum smithii, and no removal), two soil nitrogen (N) treatments (ambient N and low N, immobilized by adding 500g C m^-2 year^-1), and two winter precipitation treatments (ambient and increased by 20 mm H₂O m^-2 year^-1)  in a mixed-grass meadow just north of Boulder, CO (ele ~1500m).  In the spring of 2013, we measured species composition (point-intercept) and aboveground biomass (by functional group) in response to treatments.  We calculated diversity metrics (Shannon, Fisher’s alpha, and richness), relative competition intensity (RCI), and log response ratio (lnRR) of both grasses, and analyzed differences by treatment and species using three-way ANOVAs (R Statistical Software).

Results/Conclusions

Removal treatments effectively reduced target grasses in most cases, but B. tectorum re-established under winter wet-ambient N conditions while P. smithii grew back under winter wet-low N conditions.  Carbon additions decreased B. tectorum growth by ~60% (p<0.001) and exotic forb growth by ~35% (p=0.031); in contrast, native grasses responded positively (50% increase; p=0.007).  The main effect of B. tectorum removal was an increase in exotic forb growth while P. smithii removal increased exotic grasses.  However, we found no effect of removal treatments on diversity.  Instead, species richness (R²=0.15, p=0.001) and Fisher’s alpha (R²=0.25, p<0.001) were negatively influenced by percent exotic cover (mostly explained by exotic forbs rather than B. tectorum).  Calculated RCI and lnRR values were highly variable and did not provide evidence for B. tectorum being a superior competitor over P. smithii.  In sum, we found that B. tectorum did respond positively to the increased N deposition and wetter winters that the Front Range is currently experiencing, while native grasses and forbs were more abundant under simulated historic low N conditions.  However, our results suggest that B. tectorum may be a passenger, rather than driver, of plant community change in the Front Range.